This invention relates to a method for the synthesis of oxidic high-temperature superconductors of defined composition having the general formula SE-EA-Cu-O, wherein SE represents a rare earth metal and EA represents an alkaline earth metal, and especially those with a perovskite structure, by sintering of the starting oxides.
Since reports of the spectacular results of the IBM team of Bednorz and Mueller regarding a significant increase in the transition temperature of oxidic superconductors, intensive research activities relating to these materials have been taken up all over the world (Bednorz, J. G. and Mueller, K. A., "Possible High T.sub.C to Superconductivity in the Ba-La-Cu-O System. Condensed Matter," Zeitschr. f. Physik 64, 189-193 (1986)). Further progress in this field has been reported through late 1986 and early 1987. The principal focus of this work has been on compounds of the general formula: EQU SE.sub.1 EA.sub.2 Cu.sub.3 O.sub.7-x, X.ltoreq.0.5
wherein SE represents a rare earth metal and EA represents an alkaline earth metal. Oxidic superconductors comprise compounds of this phase system having a perovskite structure. Of particular interest are compounds within the systems La-Ba-Cu-O and Y-Ba-Cu-O.
The superconductivity of such materials and their high transition temperatures, which according to Politis et al. (Wu, M. K. et al., "Superconductivity at 93K in New Mixed Y-Ba-Cu-O Compound System at Ambient Pressure," Physical Review Letters 58, 908-910 (1987)) may well reach 0.degree. C., depend on the formation of specific perovskites with non-stoichometric (astoichiometric) oxygen contents. Moreover, superconductivity and transition temperature appear to depend, in a manner which is not yet quite clear, on the occurrence of Cu.sup.3+ -Cu.sup.2+ charge states within the perovskite lattice.
In the past, such compounds have been prepared by sintering specific homogeneous oxide mixtures at relatively high sintering temperatures, following a cold pre-pressing at elevated pressure. It has been observed that the corresponding phases, depending on the experimental conditions, are formed only to a limited extent. The quality of the product, especially as reflected by the transition temperature T.sub.C, will depend significantly on the experimental conditions.
At this time, several problems are presented with respect to preparation of such superconductors on an industrial scale. Given the present state of the art, the complex chemical structure of the product with its defined deviation from stoichiometry is difficult to cope with from a technological point of view; for example, the sintering temperatures necessary for structural compacting have a system-specific effect on both the oxygen contents and the valence states and coordination relationships of the perovskite structure. Further, the superconductive oxides of interest present major problems with respect to preparing products of a desired shape, on account of their brittleness. Thus, in addition to the work necessary for solution of fundamental problems of theory, research activities must be directed to the reduction of these scientific findings to technological practice. Techniques proposed to date for the solution of these problems do not appear suitable for the development of complex superconductive materials having the necessary quality and stability, nor do the available techniques appear economically feasible for industrial-scale production.
A survey of the literature confirms current understanding that the physical phenomena of superconductivity at present still appear to depend on as yet not fully controllable textural and structural properties of a material. In particular, depending on the concentration and distribution of secondary phases which accumulate at the grain boundaries of polycrystalline materials, the overall properties of the product may be dramatically affected; these "detrimental" grain boundary segregations may not, however, necessarily be involved in the overall mechanism. In addition, structural density seems to influence the quality of the superconductor, especially the stability of the product and permissible current densities in the use thereof.